Ultrasonic transducers are used in many cases, where a desired action can be obtained with the power of an ultrasonic wave or vibration. One example is constituted by cleaning equipment, in which a cleaning fluid is excited to ultrasonic vibrations by an ultrasonic transducer enabling firmly adhering impurities and contaminants to be removed from the surface of immersed objects. Another example is provided by ultrasonic welding equipment, in which use is made of the dissipated heat occurring in the case of high intensity ultrasonic vibrations in elastic materials. In electronics engineering, ultrasonic transducers are known e.g. for emitting directed, modulated ultrasonic waves in water, which can be used for intelligence transmission or for the position finding of ships under water. In the medical field, ultrasonic transducers e.g. are used for shattering renal and urinary calculi, because high intensity ultrasonic waves can be relatively easily led by waveguides through natural body openings (e.g. the urethra) to the calculi to be destroyed, so that the latter can be removed without major operations being necessary.
Ultrasonic transducers are termed as electromechanical transducers and can be constructed according to different principles. In the case of high power levels, preference is given to the use of ceramic disks, which operate according to the inverse piezoelectric effect principle.
In operation, ultrasonic transducers are coupled to a mechanical resonance arrangement. However, high power outputs with a good efficiency can only be achieved in all cases, if the arrangement operates precisely at the resonant frequency of the mechanical arrangement, or very close to it. The electrical power supplied to the ultrasonic transducer, must therefore be very accurately supplied with the operating frequency of the mechanical arrangement. However, as the resonant frequency of the arrangement can change in operation, i.e. due to heating, load changes or ageing phenomena during its life, the power generator which controls the ultrasonic transducer must be regulated on the basis of the resonance conditions of the mechanical arrangement. For this purpose use is made of phase measuring devices, which measure the phase difference between the voltage applied to the ultrasonic transducer and the applied current. The output signal of the phase measuring device is used to finely regulate the power generator frequency in such a way that voltage and current at the ultrasonic transducer maintain a predetermined phase displacement, i.e. are generally as far as possible in phase.
A known arrangement of this type is described in "The High-Frequency Generator for the Ultrasonic Lithrotrite", IN-SIGHT, Urology Edition April 1983, published by Karl Storz Endoscopy-America Inc., Culver City, CA/USA. This is in fact a high frequency power generator, which supplies an ultrasonic probe for the shattering of urinary calculi. The high frequency generator contains a voltage-controlled oscillator or VCO, which can be frequency-tuned with an applied voltage. The high frequency output voltage of the oscillator is amplified by a power amplifier and supplied to the ultrasonic transducer. At the output of the amplifier, a phase measuring device measures the phase difference between the current flowing through the ultrasonic transducer and the voltage applied to the ultrasonic transducer and transfers the output signal to the VCO, so that the frequency of the latter is continuously finely tuned to the resonant condition.
However, this method does not operate in an optimum manner. It is known from the use of the described power generator, that the oscillation or vibration of the ultrasonic transducer sometimes "breaks off", i.e. despite the resonance regulation, the VCO operates at a completely incorrect frequency and the ultrasonic transducer no longer supplies any power. This behaviour is particularly observed if there is a fluctuation in the ultrasonic transducer loading, e.g. if the operating surgeon presses the probe connected to the transducer too strongly against a urinary calculus to be destroyed. It is also known from the production process of the described power generator, that it is not possible during the final phase regulation to set a zero phase displacement between the ultrasonic transducer current and voltage, although this would appear to be the optimum solution. When setting the phase displacement, which constantly regulates the power generator in operation, it is often necessary on each occasion to find a compromise between the maximum power output and the stability of the output power. However, it is never possible to simultaneously achieve a maximum possible power and absolute stability of the vibration against load fluctuations.
There are a number of causes for this. Firstly an ultrasonic transducer has an equivalent circuit diagram, as a series resonance circuit is built up from a capacitor, an inductance coil and an ohmic resistor. A capacitor, namely the so-called rest capacitor of the ultrasonic transducer is in parallel with this series resonance circuit. The value of the ohmic resistor in the series resonance circuit changes with varying loading, being smallest with the smallest loading and rising as the load rises.
Furthermore, the ultrasonic transducer is spatially separated from the power generator, e.g in the case of ultrasonic urinary calculus shattering. The spacing must be bridged by a lead, whose capacitance gives a further parallel capacitance to the ultrasonic transducer. Thus, the electrical impedance of the complete ultrasonic transducer including lead connected to the power generator no longer has the phase angle zero at the actual resonance of the mechanical arrangement. At the resonant frequency of the mechanical arrangement, the phase angle of the electrical impedance does not even have a constant value if the load changes. Thus, a fixed setting of the phase regulation can at the best give the optimum power output for a single loading value. For any other loading, the VCO phase regulation influences it in such a way that to a greater or lesser extent the frequency differs from the resonance of the mechanical arrangement.
The locus of the input impedance of an ultrasonic transducer with rest capacitance and lead capacitance is not symmetrical to the real axis in the resonant frequency range. The greater the loading the more asymmetrical it becomes and can migrate to such an extent in the capacitive half-plane that there is no value at all in the vicinity of the resonant loop at which the phase between the current and the voltage has the value to which the phase regulation is set. With such a loading, the vibration or oscillation breaks off and the ultrasonic transducer no longer supplies any power.